The recent proliferation of inexpensive infrared imaging systems for battlefield detection and targeting has created a new threat for the US Armed Forces. These imaging systems, often referred to as FLIRs (forward looking infrared), can operate in wavelengths from 1 to 20 microns with sensitivities of a few tenths of a degree centigrade. They operate by detecting the contrast of infrared radiation between a target and its background, principally in the 3-5 and 8-12 micron regions. Engines, generators, and other heat producing devices are readily visible to an infrared detector. Thermal radiation to areas surrounding the engine, generator, or heat producing device also enhances the ease of detection, since the apparent size of the heated surface increases considerably. Other effects, such as solar loading and friction lead to surface temperatures higher than the background. Human skin also emits more radiation than the typical background, making military personnel and their war machines more easily detectable with infrared devices. A need exists for defeating these IR imaging systems by making it difficult for the device to discriminate between the potential target object and its background. In addition, since the human eye remains the most sensitive instrument for detecting objects in the visual region (0.3-0.7 microns) and electro-optic sensors are available to discriminate colors, the need also exists for defeating these IR imaging devices without detracting from conventional visual camouflage.
The ability of a material to emit infrared radiation is called its emissivity. Two materials at the same thermal temperature but with one having a high emissivity and one having a low emissivity will not radiate the same amount of infrared energy. The high emissivity material will radiate strongly while the low emissivity material will radiate very little. A thermally warm object can be made to appear cooler to a FLIR if its surface is made reflective to background infrared radiation by use of a low emissivity material. The relationship between emissivity and reflectivity for infrared radiation is that low emissivity materials tend to be highly reflective and that high emissivity materials tend to be very poorly reflective. Emissivity is a fundamental property of the electron mobility found in the material. Thus, a good electrical conductor, such as a metal, with high electron mobility has a low emissivity and is very reflective. A poor electrical conductor, such as glass or organic materials, has little electron mobility, is a very poor reflector, and has a high emissivity. In general, the sky and surrounding environment are thermally much cooler than the object to be camouflaged in the infrared. A reflective surface will reflect into the FLIR the lower level of radiation coming from the sky and the surrounding environment, masking the temperature of the object by making it appear to be thermally cooler.
The formulation of a practical coating suitable for both infrared and visible camouflage is difficult to achieve with available materials. Many metallic pigments such as aluminum, metallized mica flakes or other finely powdered metals can be incorporated as pigments into paints to achieve various levels of infrared reflectivity. The influence on the final coating emissivity from the individual component is dependent on the amount of metallic pigment included in the coating. Incorporation of high levels of low emissivity pigments to create a low emissivity coating lead to poor coating physical properties incapable of withstanding the rigors of combat deployment. As all metallic pigmented coatings appear to the eye as off-white to gray in color, further addition of high emissivity visual pigments to allow visual camouflage coatings both increase the coating emissivity and decrease the coating physical properties. The requirement for combined visual and infrared camouflage for aircraft, armor, and soldiers cannot presently be met with conventional coating materials.
Metallized cenospheres as taught in U.S. Pat. No. 4,624,798 are an unconventional type of highly effective pigment material. These unique materials enable preparation of compositions with high IR reflectivity. The size and shape of the spheres allow for low emissive coatings without high pigment loadings and subsequent loss of the coating physical properties. In addition, the diffuse spheres integrate the infrared radiation from the background to provide an apparent infrared match with the surroundings. Due to the spherical shape, a uniform surface area is presented in a coating, providing greater influence on the emissivity of the final coating.
I have discovered a novel technique to incorporate color on metallic surfaces in a way which does not appreciably interfere with their ability to control infrared reflectivity. Many commercially available pigments can be burnished into a metallic surface to yield a modified surface which retains the color of the pigment, and in addition still retains very high infrared reflectivity and low emissivity. (NOTE: I use the word xe2x80x9cburnishxe2x80x9d in this application to refer to an impregnation process in which pigment particles are mechanically forced into a metallic surface by abrasive and impact methods.) This burnished surface technique holds the pigment particles so strongly that the pigment cannot be removed by normal washing or by solvents. The amount of pigment burnished into the surface may be varied to achieve various color saturations. All commercially available pigments are not suitable for burnishing. Important attributes for those pigments that perform well include the hardness of the pigment particles and a dispersed particle size of less than 1 micron.
I have shown that colored pigment particles, less than 1 micron in size and mechanically attached to a metallic surface by burnishing, produce colored materials that retain excellent infrared reflective properties. The colored pigment particle, with dimensions much smaller than the wavelength of the infrared radiation, affects the infrared reflectivity of the metallic substrate only slightly. At the same time, the dimensions of the pigment particle attached to the metallic surface are similar to or larger than visible wavelengths and allow a strong contribution to the visual color.
Metallic sheets, which may consist of metal plate or a metal plated substrate, may readily be colored to produce a low emissivity surface having the selected hue. The method consists of grinding or burnishing the pigment onto the surface with a burnishing tool, which may be hand powered or machine powered, and removing any excess pigment.
Burnishing of metal particles or metal-coated particles with colored pigments can be accomplished by several techniques. The preferred technique is to place the metallic particles in a vibratory tumbler with an appropriate amount of colored pigment, and vibrate the mixture from one-half to four hours. The rubbing and grinding motions in the vibratory tumbler enable the pigment to be burnished into the metallic surface. The mixture is then sieved to separate the products, and the resulting controlled reflectivity color particles can be utilized directly in a coating formulation which can be applied to metal, plastic, composite, or fabric substrates to yield products which have the desired infrared reflectivity and the desired visual color. Another technique is to tumble the metallic particles with the pigment and a larger grinding media in a conventional ball mill for a period up to 18 hours. This method requires screening and washing of the final product. The color saturation of the resulting metallic particles can readily be varied by the length of burnishing time and by the amount of pigment used. The pigment particles are mechanically bound to the surface of the metal and are not easily removable by washing or handling. The pigment particles remain on the metallic surface when the colored particles are utilized in coating formulations.
The preferred method of producing colored highly reflective infrared pigments is accomplished by using a vibratory tumbler similar to the Ultra-Vibe 18 or Ultra-Vibe 45 manufactured by Tru Square Metal Products, Auburn, Wash. The tumbler is charged with the metallic or metal-coated particles to be colored. Colored pigment is added and the tumbler is operated for xc2xd-12 hours. After the prescribed time, the particles are emptied onto a 100-150 micron screen and the colored metallic particles are sieved to remove clumped and very fine material.
To evaluate the infrared properties, reflectivity and emissivity measurements were performed using an Inframetrics 760 Reflectometer at normal incidence. Neat pigment measurements were performed in the following manner:
A 12xe2x80x3xc3x9712xe2x80x3xc3x97xc2xcxe2x80x3 aluminum plate was placed on the surface of a 12xe2x80x3xc3x9712xe2x80x3 flat hotplate heated to 100xc2x0 C. One 6xe2x80x3 square corner of the aluminum plate was painted black to represent a high emissivity material, approximately 0.95. The rest of the plate was polished aluminum to represent a low emissivity material, approximately 0.09. The pigment to be measured was placed in a 3xe2x80x3 diameter aluminum weighing dish filled to a uniform height of xe2x85x9xe2x80x3 and completely covering the bottom surface of the dish. The dish was placed on the polished aluminum panel for 15 minutes to achieve thermal equilibrium. The Inframetrics 760 was then used to measure the apparent temperatures of the three different materials. The apparent temperature of the sample could then be converted to emissivity.
Measurements were similarly performed on applied coatings by placing a 6xe2x80x3xc3x976xe2x80x3xc3x97xe2x85x9xe2x80x3 aluminum panel coated with an infrared reflective composition incorporating materials produced by the method described above onto the 12xe2x80x3xc3x9712xe2x80x3xc3x97xc2xcxe2x80x3 aluminum plate situated on the hot plate. The coated aluminum panel was allowed to thermally equilibrate with the heated hot plate for 15 minutes. The Inframetrics 760 was then used to measure the emissivity of the coated panel as described above.